I like to think the six human souls came back as ghosts and hang out where they're bodies are (the coffin room) and discussing about what is happening and how much longer till they can get the heck out the underground

Then when they see the seventh human (Frisk) they start freaking out and like "OMG THIS HUMAN IS ALIVE" and all have awkward explanations for how they died

Yeah, I'll make a short story bout them soon